This invention relates to a cylinder lock with key, incorporating a picking preventing means.
As is known, cylinder locks generally comprise a plug carried rotatably within a seat formed in the lock cylinder or body and being rotatively connected to a so-called bit. The plug has, on a radial plane thereof, a set of holes which accommodate pins of a shorter length than the holes and adapted to cooperate with respective tumbler pins accommodated in a second set of holes formed in the lock cylinder and being spring biased.
With the key removed, owing to the bias force of the springs, some pins are allowed to partly penetrate the pin holes to inhibit rotation of the plug.
The plug holes are put into communication with the key slot, and the notches of the key are related to the length of the pins such that, on inserting the key, the contact points between the pins and mating tumbler pins will lie on the shearing plane between the plug and cylinder of the lock, thus allowing the plug, and hence the bit, to be rotated.
With prior cylinder locks, picking efforts are mainly directed to surreptitiously produce alignment on the shearing plane of the plug pins and tumbler pins using such special implements as picking tools. Attempts of this kind are favored by the key slot being coplanar with the pin holes, thereby permitting the insertion of a lock picking tool through the key slot, enabling one to act axially on the pins and selectively move them over a sufficient distance to allow the plug to be turned.
In order to resist tampering as far as possible, it has been proposed to provide a cylinder lock having a key slot with suitably offset and shaped longitudinal ribs to interfere with pin manipulation. However, due to reasons of manufacture, and mechanical strength requirements of the key itself, the key slot is still made too large to effectively discourage such picking efforts as described above.
From the U.S. Pat. No. 2,049,548 it is also known to provide a longitudinal rib in the key slot on which the pins of the plug rest with their inner ends. However, also this lock is susceptible to pin manipulation since picking tools may be introduced from the side under the pins and raise the latter in the plug rotating position.